The Iola register. [volume] (Iola, Allen County, Kansas) 1875-1902, 1887-05-13

mals-The Birds Third EyeIid-Union Of
the Simple and Compound In Insects.
Amidst all the marvels which the
world of nature offers to the reflective
and observant mind, there are few
which surpass in interest the wonders
revealed by a study of the mechanism
of sight in the animal kingdom. An
exhaustive study of comparative anat-
ony is by no means necessary to real
ize these wonders; in fact, we require
to be reflective as regards obvious and
familiar details, rather than learned in
scientific terms and anatomical discov-
cries. No very extensive knowledge
of the structure of the eye. for instance.
is necessary to grasp tho full signifi-
canoe of the fact that the views obtain
able from the top of st. Paul's Cathe-
drai or the dizzy pinnacles of the Alps
enter the eye through an aperture of
about an eigth of an inch diameter,
and are reproduced in the interior of
the eye on a surface averaging the size
of a sixpence, from which the size.
shape, color. position, and general
peculiarly of every object within range
are accurately conveyed to the brain.
This is only one of the ordinary won
ders of the human eye. but shares the
fate of many others in that it seldom
occupies a moments thought-no more
thought, in fact, than we bestow upon
the varieties of sight in the lower ami
mails and the lessons We may learn
from them.
Most of us, in these DAYS of cheap
science manuals. are familiar with the
general structure of our own eyes and
the rudimcntary principles of optical
science: and we are so accustomed to
look upon this particular plan of con-
strnction rs the only one by which
vision can be accomplished. that it is
impossible not 10 feel astonished when
observation discloses the extraordinary
variety of structural arrangements
which are to be found in the descend
ing scale of animals. In completely
and minuteness of detail, the eve of
man and the higher animals surpasses
all others; Yet its structure may be said
To explain itself. and to be compara-
lively simple plan. when regarded in
connection with the laws of optics. 1t
is exactly such as might be imagined
as consequence of known laws of
light transmission and known proper
tics Of matter. A first glimpse, there
fore, into the structure of the eyes of
the lower or invertebratc animals is
somewhat pcrpleXing. owing to the ar
parent contradictions met with; for the
seemingly simple plan of the mammal
eye is lost in diversity of external
form and internal structure which is
truly marvellous; in some cases, the
elements considered to be essential are
apparently missing; and in others. adj
<.i:ions are found which have no coun-
terpart in eyes supposed to be more
perfect.
A little reflection will, however, show
that these divcrsities of structure,
wherever met with, are necessary to
the particular mode of life which their
possessor was designed to lead in the
ranks of creation. In almost all man,
mails for instance, below the Primates.
:, third eyclid-or. as it is called, the
"'nictitating' membrane = is found,
commodiously folded up in the inner
corner of the eyeball, ready at any in
slant to sweep across its surface. This
is found in the eyes of all birds. and of
such quadruples as require it, its use
being to wash the eye free from any
foreign particles with the help Of the
lachrymal humor, and to defend it
from sudden injuries. 1t is not alto
gether an opaque membrane. and there
fore. when brought into play by birds
flashing through the air. or flying
straight upwards in the full glare Of |
the rays of the sun, it prevents the en-
trance of dust or other injurious mat
ter, protects the eye from too strong a |
light, and at the same time does not |
interfere with sight The motion of |
this membrane is performed in Curl
Gus manner; there are not two antago-
nistic muscles, one pulling forward |
and the other backward, as
one might imagine. but only i
one muscle, attached by a
tendon to the membrane. The nicti- |
Eating membrane itself is an elastic |
substance. capable of being drawn out
by force like piece of elastic. and re-
turning to its former position when the |
force is removed. The muscle which |
this force is one of the most |
marvellous mechanisms to be found in
nature. Placed at the back of the eye.
it is passed through loop formed by
another muscle, and there infected as if |
1t we're round a pulley. This peculiar
ity. accessory for the rapid action